Do not hide expression when user presses Enter or = (#695)

* #653 Do not hide expression on EQU operation

* #653 Clear expression, when user makes an action

* #653 Fixed UI tests

* #653 Review fixes

src/CalcManager/Header Files/CalcEngine.h

@@ -176,6 +176,7 @@ private:
     void DisplayAnnounceBinaryOperator();
     void SetPrimaryDisplay(const std::wstring& szText, bool isError = false);
     void ClearTemporaryValues();
+    void ClearDisplay();
     CalcEngine::Rational TruncateNumForIntMath(CalcEngine::Rational const& rat);
     CalcEngine::Rational SciCalcFunctions(CalcEngine::Rational const& rat, uint32_t op);
     CalcEngine::Rational DoOperation(int operation, CalcEngine::Rational const& lhs, CalcEngine::Rational const& rhs);